A) Field of the Invention
The present invention relates to a semiconductor light emitting device and its manufacture method.
B) Description of the Related Art
Since the advent of high outputs of light emitting diodes (LEDs), the use field of LEDs is expanding day after day to various fields ranging from a device indicator lamp, an outdoor display lamp, backlight illumination for a liquid crystal display, to indoor illumination. In these markets, LEDs are desired to have still higher outputs.
High outputs of LEDs have been realized by improvement on an internal quantum efficiency, improvement on a light extraction efficiency, reduction in a package loss, improvement on device heat radiation and the like.
A high output of a white LED is strongly desired nowadays. However, an internal quantum efficiency of InGaN based LEDs as light sources has reached 70%, and improvement in this direction is considered to be approaching its limit. Therefore, improvement on the light extraction efficiency has been extensively studied.
FIGS. 9A and 9B are schematic cross sectional views of an LED of a flip-chip structure (a structure in which light is output from a side opposite to a substrate surface on which an LED optical emission layer is formed).
Reference is made to FIG. 9A. On a sapphire substrate 20, a nitride semiconductor layer 21 is formed which is a semiconductor optical emission layer containing InGaN. Formed on the nitride semiconductor layer 21 are an n-side electrode 22 for supplying electrons to the nitride semiconductor layer 21 and a p-side electrode 23 for supplying holes to the nitride electrode layer. An insulating protective film 24 is formed surrounding both the electrodes 22 and 23.
The n- and p-side electrodes 22 and 23 are connected to n- and p-side lead electrodes 34 and 32 respectively via conductive connection members 33. Both the lead electrodes 32 and 34 are formed on an insulating film 31 formed on a support substrate 30.
The p-side electrode 23 is made of material such as Ag having a high optical reflectivity, and reflects directly or indirectly light emitted from the nitride semiconductor layer 21. Reflected light is output from the sapphire substrate 20 side.
The sapphire substrate 20 is transparent with respect to light emitted from the nitride semiconductor layer 21. Since its optical absorption loss is small, the sapphire substrate, for example, is used as a good window member.
Heat dissipation can be enhanced by bonding the support substrate 30 to a sub-mount, a frame, a stem, a heat sink, a wiring substrate and the like. Large current and high output operations are therefore possible.
Reference is made to FIG. 9B. FIG. 9B is a cross sectional view taken along the line 9B-9B shown in FIG. 9A. The p-side electrode 23 is disposed generally in a central area of LED so that light from the nitride semiconductor layer 21 can be reflected efficiently.
FIG. 10 illustrates optical output paths of light emitted from LED of the flip-chip structure shown in FIGS. 9A and 9B.
In FIG. 10, the structure of the nitride semiconductor layer 21 is shown in more detail than FIG. 9A, for the convenience of explanation. The nitride semiconductor layer 21 includes an optical emission layer 41 for emitting light through recombination of holes and electrons and n-type and p-type nitride semiconductor layers 40 and 42, respectively, sandwiching the optical emission layer 41. The n-type nitride semiconductor layer 40 is electrically connected to the n-side electrode, and the p-type nitride semiconductor layer 42 is electrically connected to the p-side electrode 23.
Optical output paths are classified into four types: paths formed by direct light emitted from the optical emission layer 41 directly toward the sapphire substrate 20 side; paths formed by reflection light reflected by the p-side electrode 23 and emitted toward the sapphire substrate 20 side (these two lights are referred to as “front emission light”); paths formed by substrate end face emission light emitted from an end face (side end face) of the sapphire substrate 20; and paths formed by nitride semiconductor layer end face emission light emitted from an end face (side end face) of the nitride semiconductor layer 21.
The nitride semiconductor layer 21 contains GaN crystal. The refractive index of GaN crystal around an emission wavelength of 470 nm (in vacuum) is about 2.4, and a refractive index of sapphire is about 1.77. Therefore, the total reflection angle of light emitted from the optical emission layer 41 and propagating from the GaN layer to the sapphire substrate 20 is about 47.5°. It is herein assumed that the optical reflectivity of the p-side electrode 23 is 100%, the optical absorption factor of the nitride semiconductor layer 21 is 0%, and light emitted from one point of the optical emission layer 41 radiates omnidirectionally at an equal strength. In this case, calculations show that light capable of entering the sapphire substrate 20 is about 32.4% and light reflected at an interface between the sapphire substrate 20 and compound semiconductor layer 21 is 67.6%. Light that is unable to enter the sapphire substrate 20 becomes light propagating (being guided) in the nitride semiconductor layer 21 (hereinafter referred to as “propagation light”).
The propagation light is reflected at an interface between the p-type nitride semiconductor layer 42 and p-side electrode 23 and at an interface between the sapphire substrate 20 and n-type nitride semiconductor layer 40, and propagates through the optical emission layer 41 between two reflections. This propagation light attenuates because of an optical reflection loss at a reflection surface and an inter-band optical absorption in the nitride semiconductor layer 21. The propagation light attenuates also because of optical absorption caused by other reasons (optical absorption at non-radiative centers of nitride semiconductor crystal itself and by crystal defects).
An LED having the structure shown in FIGS. 9A and 9B and manufactured in a size of about 1 mm2 possesses some problems, such as a lowered light extraction efficiency, an inhomogeneous optical emission, a lowered power efficiency, an increased heat generation and the like.
The light extraction efficiency lowers as emission light from a nitride semiconductor layer end face (side end face) is reduced, for example, by attenuation by optical absorption by crystal defects in the nitride semiconductor layer (if a buffer layer to be described later is formed, optical absorption in the buffer layer is also included), by a reflection loss at an interface between the p-side electrode and p-type nitride semiconductor layer, by re-absorption in the optical emission layer and the like.
The inhomogeneous optical emission is caused if current supplied from the n-side electrode does not diffuse uniformly in the n-type nitride semiconductor layer because of a large device size, but flows more in a region near the electrode.
The lowered power efficiency is caused, for example, by a power loss (power transmission loss) caused by a longer distance of current supplied from the n-side electrode and flowing in the n-type nitride semiconductor layer. Further, since the power loss is transformed into heat, the heat generation becomes large. The power loss is generated also by an increase in resistance components caused by narrowing a relative electrode area, by a lowered optical emission efficiency due to uneven current distribution, and by other reasons.
FIG. 11 is a schematic cross sectional view showing an LED lamp. The LED lamp shown comprises semiconductor light emitting diodes 300 shown in FIGS. 9A and 9B.
The semiconductor light emitting diode 300 is placed on a lamp substrate 303. p- and n-side lead electrodes of the semiconductor light emitting diode 300 are connected to p- and n-side lamp lead electrodes 304 and 305 to receive an electric power from an external power source.
The lamp substrate 303 has a reflection horn 301 disposed around the semiconductor light emitting diode 300. The semiconductor light emitting diode 300 and reflection horn 301 are covered with translucent resin 302 on the lamp substrate 303.
As understood from the description made with reference to FIG. 10, light emitted in the semiconductor light emitting diode 300 is output from the sapphire substrate surface of the semiconductor light emitting diode 300 and from an end face (side face) of the semiconductor light emitting diode 300. In order to increase the optical output, light output from the end face (side face) of the semiconductor light emitting diode 300 is guided to the upper surface of the LED lamp by using the reflection horn 301, for example.
Since the reflection horn 301 is disposed whose shape greatly influences an output and optical paths of the LED lamp, there arise some problems, such as restrictions on the size and shape of the LED lamp. Further, light output from the surface of the n-type nitride semiconductor layer on the electrode side, and light output from the optical emission layer and an end face of the p-type nitride semiconductor layer become stray light and do not contribute to improving the LED lamp output.
In order to improve the light extraction efficiency of a flip-chip LED, various structures have been proposed, such as a structure that a multi-layer reflection film is formed on an end face of an optical emission region (e.g., refer to JP-A-2002-353504), a structure that a reflection layer is formed on an end face from a semiconductor layer to a translucent substrate (e.g., refer to JP 3540605) and a structure that a reflection layer is formed on an inclined surface (e.g., refer to JP-A-2005-39197).
In order to improve the LED operation efficiency, a structure in which an n-side electrode is formed on the upper and side walls of an n-type nitride semiconductor layer, surrounding an optical emission region, has been proposed (e.g., refer to JP-A-HEI-11-150300).
JP-A-2002-353504 discloses an LED structure capable of improving the light extraction efficiency by minimizing a loss, at a mesa wall, of incident light having an angle from −10° to 30° relative to a substrate. However, this structure has a limit in the incident angle of light that can take advantages of the effect.
JP 3540605 discloses the invention of a light emitting diode having a structure in which a reflection electrode is disposed on an inclined surface extending from an n-type nitride semiconductor layer to a sapphire substrate and light emitted from an optical emission layer is reflected at the reflection electrode toward the substrate side. In this light emitting diode, since the reflection electrode is formed only on the n-type semiconductor layer and on the sapphire substrate, light leaks from the end faces of the optical emission layer and a p-type semiconductor layer.
JP-A-2005-39197 discloses an invention of a semiconductor light emitting diode having a structure in which light confined in semiconductor is output along a front side direction by using a reflection layer disposed on an inclined surface. In the semiconductor light emitting diode of this structure, since the reflection layer and an n-side electrode are formed separately, a space exists between the n-side electrode and reflection layer and there is leak light.
JP-A-HEI-11-150300 discloses an invention of a nitride semiconductor device presenting a high optical emission efficiency when current is injected into an optical emission layer. The optical emission efficiency is improved by broadening a contact area between the semiconductor layer and an electrode, but a light extraction efficiency is not improved.